[#47873081] Added column private to the questions table
[#47873081] Extracted the `Mark as Identifier` and the file upload fields into
partials.
[#47873081] Show `Mark as Private` for each question.
[#47873081] ExcelData filters out private questions if a flag is passed in.
[#47873081] Add an option for filtering out private questions in the excel dialog.
[#47873081] Pass a flag to filter private questions to the ResponseController.
[#47873081] Modify the controller spec so it sends booleans as strings.

- Because that's how the params would come in from a browser.
[#47873081] Add a default value (false) to the `private` column in `questions`.
[#47873081] Add a Reports::Excel::Questions class which performs filtering.
[#47873081] ExcelData gets its questions from an instance of Reports::Excel::Questions.
Add the Fuubar formatter.
[#47873081] Reports::Excel::Responses#build takes an "optional" options hash.
[#47873081] Reports::Excel::Questions filters out private questions by default.
[#47873081] ResponsesController passes in the `current_ability` to the Questions wrapper.
[#47873081] Change `Filter private questions` to `Include private questions?`
[#47873081] Move the filtering in Reports::Excel::Questions to a scope.
[#47873081] Add abilities for changing excel filters.

- Designer and cso_admin can change excel filters for surveys created by their organisation.
- Other roles can't change filters at all.
- No one can change filters for surveys shared with their organisation.
- Super Admin can change filters for all surveys.
[#47873081] Show the excel settings only if the user has permission to see the private questions.
[#47873081] A CSO Admin can see private questions for a survey shared with his organization.